The present invention relates to x-rays and, more specifically, to X-ray focussing using a refractive X-ray arrangement. The refractive arrangement for X-rays comprises a member of low-Z material, said part of low-Z material having a first end adapted to receive x-rays emitted from an x-ray source and a second end from which emerge said x-rays received at said first end, and first and second surfaces. The invention also concerns a lens and a method for manufacturing the arrangement.
With the advent of 3rd generation synchrotron x-ray sources, hard x-ray optics is a field of growing interest with applications in research, material testing, chemical analysis and medical imaging and therapy. Prior art focusing elements in this energy region use the methods of reflection and diffraction, e.g. best crystals, curved mirrors, Fresnel zone plates and capillary optics. These elements are generally expensive and technologically challenging to manufacture, limiting their use in commercial-grade applications.
Another shortcoming associated with prior art high-energy x-ray focusing techniques, such prior art attempts are limited to generating a single-peak energy distribution. Hence, such experimental methods are not well suited to applications requiring more than one x-ray energy peak, such as dual-energy x-ray imaging.
It is well known that the refractive index of any material can be expressed by
n=1xe2x88x92xcex4xe2x88x92ixcex2xe2x80x83xe2x80x83(1)
Refractive lenses can easily be fabricated for use in the visible light region, since materials having a refractive index n far from unity and a small absorption in this region are readily available. In contrast, optical elements utilizing refraction are intrinsically difficult to fabricate for use in the x-ray region, since in this energy region, all materials have an index of refraction n near unity and exhibit a large absorption. Consider a concave piece of material having a circular revolution with the radius of curvature R. Such a piece of material will focus a plane-wave entering parallel to the axis at a focal distance of f. The focal length is given by                     f        =                  R          δ                                    (        2        )            
A lens fabricated according to eq. 2 would have a very large focal length, since d is typically 10xe2x88x925 or 10xe2x88x926 in the hard x-ray region. Examples of such lenses were given by Suehiro et al (Nature 352 (1991), pp. 385-386). In a correspondence, this approach was ruled out for any practical application by Michette (Nature 353 (1991), p. 510). The extent to which the focal length can be shortened by reducing R has limitations in terms of fabrication technology and practical use.
A significant improvement was achieved when Snigirev et at (Nature 384 (1996), pp. 49-51) cascaded N drilled holes in a piece of aluminum. This corresponds to 2N concave surfaces, thereby reducing the focal length by the same factor. The total focal length of the compound lens is given by                     F        =                              f                          2              ⁢              N                                =                      R                          2              ⁢              δ              ⁢                              xe2x80x83                            ⁢              N                                                          (        3        )            
This lens still suffered from spherical aberration and high absorption and focusing was only achieved in one dimension. These shortcomings have been addressed by several authors. Similar solutions are also known through U.S. Pat. No. 5,594,773 and U.S. Pat. No. 5,684,852.
Low-Z materials have been used for decreased absorption and two-dimensional focusing has been achieved by, e.g. Elleaume Nucl. Instr. and Meth. A 412 (1998), pp. 483-506) by means of crossing two linear arrays.
Another lens is described in a U.S.A Patent Application entitled xe2x80x9cA COMPOUND REFRACTIVE X-RAY LENSxe2x80x9d, now U.S. Pat. No. 6,091,798, which discloses a novel manufacturing technique to make parabolic profiles by splitting the lens in two halves at the symmetry axis, thereby reducing spherical aberration and absorption.
However, aberration free compound reflective x-ray lenses still rely on elaborate and expensive manufacturing techniques. Hence, such refractive lenses are not well suited to commercial-grade applications. Furthermore, such prior art refractive x-ray lenses are limited to generating a single-peak energy distribution. As yet another disadvantage, prior art refractive x-ray lenses have, for a given energy, a fixed focal length, which cannot be varied.
Thus, a need exists for a refractive x-ray lens, which is well suited for commercial applications and which does not suffer from the disadvantageous inherited by the known lense. Still another need exists for a refractive x-ray lens, which is able to generate a dual energy distribution from an x-ray source. Yet another need exists for a refractive x-ray lens for which the focal length for a given energy can easily be varied. Still, another need exists for a high-energy x-ray leas able to generate a dual energy distribution from a broadband x-ray source.
A further need exists for a method readily to form a refractive x-ray lens at a low cost, e.g. so that high-energy x-ray optics should find its way from specialized research facilities into general applications in industry and commercial RandD.
The present invention provides an x-ray lens which is well suited for commercial applications. The present invention further provides a method readily to form a compound refractive x-ray lens. The present invention also provides a refractive x-ray lens able to generate a dual-energy distribution from a broad energy x-ray source. Furthermore, the present invention provides an x-ray lens for which the focal length for a given energy can easily be varied. The present invention achieves the above accomplishments with a novel x-ray focusing apparatus, novel x-ray lens formation methods and novel methods for focusing of x-rays.
Moreover, the present invention has as an objective to increase the flux on a scanned slit. For these reasons, the initially mentioned refractive arrangement for X-rays further comprises a plurality of substantially sawtooth formed grooves disposed between said first and second ends on at least one of said first or second surfaces. Said plurality of grooves oriented such that said x-rays which are received at said first end, pass through said member of low-Z material and said plurality of grooves, and emerge from said second end, are refracted to a focal point.
Preferably, said member of low-Z material consists of a plastic material, specially one of from the group comprising polymethylmethacrylate, vinyl and PVC. It may also consist of beryllium.
Preferably, said grooves have the form of sawteeth with substantially straight cuts.
In an advantageous embodiment said pluralities of grooves have varying sizes, decreasing or increasing continuously from said first end towards said second end.
The refractive X-ray lens according to the invention comprises a volume of low-Z material, said volume having a first end adapted to receive x-rays emitted from an x-ray source and a second end from which emerge said x-rays received at said first end and first and second surfaces. The volume further comprises a plurality of substantially sawtooth formed grooves disposed between said first and second ends on at least one of said at least two surface, said plurality of grooves oriented such that said x-rays which are received at said first end, pass through said volume of low-Z material and said plurality of grooves, and emerge from said second end, are refracted to a focal point.
In one advantageous embodiment the lens comprises two volumes arranged such that the surfaces with the plurality of grooves are facing each other. Preferably, said two volumes each have a tilt angle to an optical axis of said X-ray. Said volumes have non-coincident focal points.
Preferably, a focal length of each of the two volumes of the lens is varied by separately varying each tilt angle.
Said volume of low-Z material consists of a plastic material, specially one from the group comprising polymethylmethacrylate, vinyl and PVC or said volume of low-Z material consists of beryllium.
Moreover, the invention concerns an X-ray system and a method for two-dimensional focusing of X-rays and including at least two leases according to above. The focusing is obtained by arranging said at least two lenses, such that each x-ray traverses both of lenses in sequence and that one of said at least two lenses are rotated around an optical axis with respect to the other lens.
In one preferred application said refractive lens is coupled to at least one second commercial-grade compound refractive x-ray lens such that an array of compound refractive x-ray lenses is formed.
The method of fabricating the saw-tooth profile refractive x-ray lens is characterized by: transferring shapes of grooves onto a carrier by means of an engraving arrangement producing a master, and using said master to pressing grooves on a suitable material.